1. Field of the Invention
The present invention relates, in general, to microwave and lower frequency resonators, and, more particularly, to resonators used in electron paramagnetic resonance (EPR) spectroscopy.
2. Statement of the Problem
Electron paramagnetic resonance (EPR) spectroscopy and electron spin resonance (ESR) spectroscopy are generally used to study molecular structure in chemistry, physics, biology, and medicine. EPR is also used to determine electron wave functions, lifetimes, and impurities in dielectrics used in solid state sciences. Prior EPP spectrometers comprise four main components: 1) a magnet to provide a steady DC magnetic field; 2) a high-Q microwave resonator in which a sample is placed; 3) a microwave bridge capable of producing an oscillating electromagnetic field which is coupled via a waveguide, coaxial cable, or the like to the resonator; and 4) a signal detector with field modulation, signal amplification and display systems.
In EPR, a steady magnetic field is applied to the chemical sample in the microwave resonator. The steady magnetic field causes the electrons in the sample to precess at a frequency defined by the sample composition. The precession frequency is called the paramagnetic resonant frequency and is proportional to the intensity of the applied steady magnetic field. The precession is altered by application of high frequency energy when the frequency of the energy is near the paramagnetic resonance frequency. If the sample contains unpaired electrons, the precession change can be detected. A comparison of change in electron precession as a function of magnetic field or frequency provides valuable information relating to the chemical characteristics of the sample.
A typical EPR spectrometer uses a reflection type measurement on the electrical resonator that contains the sample. In a reflection type spectrometer a single resonator is used. The sample is placed in the resonator and microwave energy is injected via a waveguide, coaxial cable, or the like into the resonator while the sample and resonator are positioned in the steady magnetic field. A microwave device called a circulator is usually used to separate the desired EPR signal from the microwave source power. A disadvantage of the reflection mode of operation is that any portion of the microwave source power that is reflected from the resonator will interfere with the EPR signal generated in the resonator. In one type of measurement, also called "continuous wave" because the input microwave energy is applied as a continuous AC signal, the resonator is tuned to provide minimal reflection of the input energy when the input frequency is different from the paramagnetic resonance frequency.
Analysis is accomplished by sweeping the magnetic field or the microwave frequency source until the precession frequency matches the frequency of the input microwave power. When the two frequencies are the same, microwave energy is absorbed by the sample resulting in reflected energy that can be detected by the detector electronics.
One disadvantage of reflection type spectrometers for continuous wave (CW) measurements is that the EPR signal is minute compared to the magnitude of the injected microwave energy. The signal detector must detect the EPR signal while separating out the injected microwave energy. It has proved difficult to completely separate the EPR signal from the input power.
Another difficulty arises in that any parasitic reflection of the microwave source caused by improper coupling of the input power to the resonator creates significant noise in the EPR signal. In addition, the source input waveguide and the detector waveguide must be critically coupled to the resonator to prevent a large reflection of the input power that would add to the EPR signal and saturate the detector electronics.
Phase noise or noise frequency modulation of the microwave source is converted to noise amplitude modulation in the reflected signal by the resonator, creating further noise in the EPR signal. Phase noise can not be eliminated from microwave sources. It can be reduced but this results in higher costs. Since the phase noise intensity is proportional to the source intensity, it becomes more serious at higher powers. Hence, current EPR tools must be operated at low power which in turn requires larger samples. A phase or dispersion component of the reflected EPR signal is difficult or impossible to study in reflection-type spectrometers because of this phase noise.
EPR tools can also be used for pulse-type measurements such as electron spin echo (ESE). In pulse type measurements, the input energy is provided by a high power pulse rather than a continuous wave microwave source. The pulse causes a near instantaneous change in the precession and a gradual decay as the sample returns to the baseline state created by the DC magnetic field. In this type of measurement the difficulty in separating input power from the EPR signal requires a delay after the application of the input pulse before a measurement can be made. Because the energy stored in the resonator by the input pulse must "ring-down" or dissipate before a measurement can be taken, much of the ESE signal can be lost in a reflection-type spectrometer.
Two types of resonators are used in EPR spectrometry. Cavity resonators were used in early spectrometers due to their easily modeled performance, availability, and high Q. Cavity resonators are called distributed element circuits because the microwave, magnetic, and electric field are continuously distributed and mix throughout the cavity. Characteristic dimensions of cavity resonators are of the same order of magnitude as the wavelength of the electromagnetic fields used. More recently, lumped element resonators have been suggested because their dimensions can be much smaller than the wavelengths of interest.
Lumped element resonators have much less mixing of the microwave, magnetic and electric fields; each are confined largely to separate physical areas of the resonator. The area where the magnetic field is concentrated can be identified primarily as an inductor. The area where the electric field is concentrated is identified primarily as a capacitor. A term used to describe some of these lumped element resonators is "loop-gap resonators". In this case, the loop is primarily inductive and the gap is primarily capacitive. In loop-gap resonators used for spectrometers, the sample is positioned in the loop so as to interact with the magnetic field.
Most loop-gap resonators are used in the reflection type spectrometers discussed above. Conventional loop-gap resonators have low Q compared to cavity resonators, however, because the magnetic field is concentrated in the vicinity of the sample, good EPR signals can be obtained.
One means that has been tried in order to reduce the problems associated with reflection type resonators is a bimodal resonator. A bimodal resonator structure would take advantage of the fact that the EPR signal is circularly polarized. In other words, the EPP signal is a rotating vector field and is equivalent to two signals that are 90 degrees out of phase in space and time. One of these EPR signals is coupled to the input source and can be detected in the conventional reflection type spectrometers discussed above. A bimodal resonator is a structure that detects the other EPR signal that is isolated from the input source. In theory, if two uncoupled modes with microwave magnetic fields oriented 90 degrees in space could be excited in the source resonator, one mode might be used to couple microwave energy into the sample and the other to detect the EPR signal. Hence, the bimodal resonator design promises to offer superior separation of input power from the EPR signal and make the EPR spectrometer immune to noise caused by reflected input power or phase noise.
The development of a practical bimodal resonator for EPR has been sought for over 20 years. A bimodal cavity resonator was commercially available from Varian Associates, Inc., but suffered from complex and difficult tuning requirements related to the cavity resonator design. Most recently, a bimodal loop-gap resonator was investigated for EPR spectroscopy. In 1992 A. I. Tapin, James S. Hyde, and W. Froncisz published a paper entitled Bimodal Loop-Gap Resonator in the Journal of Magnetic Resonance 100, 484-490 that proposed a loop-gap resonator in which the two orthogonal EPR modes did not overlap in some regions of space but overlapped and were orthogonal in the sample-containing region. Unfortunately, a commercially viable implementation has not been produced.
A need exists for a resonator structure for EPR spectroscopy that effectively isolates the input power from the detector yet is easy to tune and inexpensive to build.
Prior EPR spectrometers have used magnetic field modulation to enhance signal-to-noise ratio. Magnetic field modulation is introduced by a modulation coil creating an oscillating magnetic field that interacts with the steady magnetic field used to cause precession in the sample. The modulation coil is excited with a high frequency current, for example 100 kHz. The field modulation causes a modulation component in the EPR signal at the 100 kHz modulation frequency. The modulation component is amplified and fed to a phase sensitive detector where it is compared to a sample of the field modulation signal. The phase sensitive detector generates a first derivative of the paramagnetic resonance signal that can be detected.
Field modulation introduces noise and causes passage effects, making it an undesirable method of extracting the EPR signal. Field modulation is difficult to introduce into the resonator structure because it requires the placement of a magnetic coil in proximity with the resonator and sample. Also, to enable the modulation field to penetrate the resonator structure at reasonable power levels, the resonator must be thinned, making the resonator structure more fragile and electrically leaky.
Another source of noise in EPR spectrometers is thermal noise that is related to the temperature of the sample. To reduce thermal noise, it is desirable to cool the resonator structure including the sample to cryogenic temperatures. For low temperature measurements, it is desirable to place the resonator and field modulation apparatus in liquid nitrogen, liquid helium, or similarly filled cryostats. Presence of the field modulation coil makes placement in the low temperature cryostat more difficult. Also, it is difficult to tune conventional resonator structures while positioned in the cryostat. What is needed is a structurally rugged resonant structure that is compatible with low temperature operation and adaptable to automatic or motorized tuning in a cryostat or similar low temperature environment.
These limitations of prior art resonator structures are a primary impediment to the application of EPR spectroscopy to biology and biomedical research. Biological and biomedical applications of EPR spectroscopy are limited by low signal-to-noise resulting from the small number of spins in the sample and instrumental sources of noise, e.g., microwave source noise, magnetic field modulation, detector noise, and, in time-domain EPR, by the dead-time of the system after the microwave pulse. Because EPR is able to detect and analyze "free radicals" and metalloenzymes either naturally occurring or used as labels or probes, overcoming these impediments to EPR spectroscopy for biological samples has major commercial and scientific significance.
A closely related technology is called electron-electron double resonance (ELDOR) spectroscopy. ELDOR has long promised improved effective resolution and the ability to analyze new relaxation parameters as compared to conventional EPR. ELDOR is used in samples that have two or more electron paramagnetic resonance frequencies. In the past, ELDOR was carried out by applying a first microwave frequency to excite a first EPR signal at the first EPR frequency as is done in conventional EPR spectroscopy. A second microwave frequency was swept through the second paramagnetic resonance frequency while the first EPR signal was monitored. The variation of the first EPR signal as the second EPR frequency was excited is the ELDOR phenomenon.
The ELDOR phenomenon was difficult to create and observe in conventional resonant structures because narrow bandwidth of the resonator allowed little separation between the first and second EPR modes. Also, all of the above limitations of prior art EPR spectrometers relating to source noise, tuning difficulty, and the like are complicated severely by excitation of the sample at two frequencies rather than one. What is needed is a resonant structure allowing broadband excitation of EPR signals and an ability to detect EPR signals at frequencies that are distant from the excitation frequencies.
3. Solution to the Problem
The above problems of the prior art are solved by an EPR spectrometer having a resonant structure that effectively isolates the input source power from the detector electronics. By substantially eliminating source phase noise as a problem in the EPR spectroscopy, operation at much higher powers and the use of less expensive sources is enabled. A high degree of isolation between two loops of the crossed-loop resonator structure of the present invention and the elimination of reflected power from the input reaching the detector allows superheterodyne detection with a very stable baseline. This eliminates the need for field modulation and the signal distortion associated with field modulation. The elimination of field modulation, phase noise, and passage effects allows true dispersion spectra (as opposed to the first derivative of the spectra) to be obtained at power levels much higher than the saturation power level of the absorption signal. A high degree of isolation in the crossed-loop resonant structure reduces the effect of resonator ring-down and significantly decreases the dead-time of the instrument in pulse type measurements.